Growth & Developmentof Skeletal Muscle

Skeletal, Striated, Voluntary Muscle

6 weeks from conception

At the end of week 3, the intra-embryonic mesenchyme differentiates into three loose aggregate pairs of mesenchyme on each side of the neural tube (Paraxial, Intermediate, Lateral)

Paraxial mesoderm differentiates into the future dermatome (dorsal surface), myotome (middle layer), and sclerotome (ventral layer), forming dermis, muscle, and connective tissue respectively.

Intermediate mesoderm, will form the future urogenital system.

Lateral mesoderm will develop into future body cavities and parts of the body wall.

The paraxial mesoderm will develop into paired cuboidal bodies, or somites. These will eventually develop into the bones (sclerotome), muscles (myotome), and dermis (dermatome) of and surrounding the axial skeleton.

Somites appear as bumps on the dorsal surface of the embryo.

At the end of week 3, 4-12 somites are present By the end of week 5, 42-44 can be counted. However, most appear between days 20-30, giving this period the title of the somite period of development.

Somites appear cranially to caudally, beginning at the occipital end. They can be counted and are used to roughly estimate the age of the embryo.

5 - 7 coccygeal pairs disappear leaving 37 pairs of somites.

Dorsal View of an Embryoat about 22 days (8 somite stage)

Somite Development

Law of Original InnervationThe myoblasts (future muscle cells) form concurrently with the spinal nerves and they migrate out from the notochord together. This results in the formation of 31 spinal nerves with associated skin, muscle, and connective tissue.

Dermatome: an area of skin receiving mesenchyme from a specific somite that is supplied by a single spinal nerve and its ganglion

6 weeks 8 weeks

Mesenchyme

Myotubules

Myoblasts

Peripheral Nuclei

Single Muscle Fibre

Myogenesis

Mesoderm – pluripotent connective tissue cell Presumptive myoblast – undergoing mitosis, mononucleated

cell incapable of fusion or contractile protein synthesis Myoblast – mononucleated cell not undergoing mitosis, cell

capable of fusion and of synthesizing myofibrillar proteins Myotubule – multinucleated cell from fusion of myoblasts, may

contain sarcomeres depending on stage. Nuclei at centre in early stages migrating to periphery as matures to muscle fibre

Muscle fibre – mature multinucleated muscle cell with myofibrils

Growth of Muscle

Muscle fibre number increases prenatally and for a short time postnatally

Fiber number doubles between 32 weeks gestation and 4 months of age

Girth and length increases continue into postnatal period

Postnatal increase in muscle girth due almost entirely to hypertrophy not hyperplasia

Fibres increase length by:– increase in # of sarcomeres (major)– increase in length of sarcomeres– length increase primarily at musculotendinous

juncton in respone to functional length

Muscle Composition FETUS - fibres small in number and widely

separated by extracellular material TERM - still small, greater number, more

closely packed ADULT - Larger diameter with little space

between them Therefore:

– decrease in sodium and chloride (extracellular)– increase in potassium (intracellular)– decrease in % water content

Changes in Muscle Compositionin Water, Sodium (Na), Chloride (Cl) and Potassium (K) as a percentage of adult levels from 13 weeks of gestation to Adulthood

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13-14 Weeks 20-22 Weeks Full Term 4-7 Months Adult

Water

Na

Cl

K

Muscle Mass Estimated from Creatinine Clearance

Creatinine levels sensitive to diet and exercise

K40 estimate of Fat Free Mass

Assumed constant proportion of potassium in Fat Free Mass

Average values acceptable, whereas indivudual estimates have considerable error due to variability in proportion of potassium in Fat Free Mass

Estimated Muscle Massas a Percentage of Body Weight (data collected from various sources)

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5 7 9 11 13 13.5 15 15.5 17 17.5 20-29

Age (yrs)

% M

usc

le M

ass

Male

Female

Strength Differences

No gender difference in strength if expressed per unit of cross-sectional area of muscle

Disproportionate strength increase in male adolescence more in upper extremities than trunk or lower extremities

No significant difference (7-17 years) in lower extremity strength after adjusting for height, between boys and girls

Peak Strength Velocityoccurs after Peak Height Velocity

Muscle Width(distance and velocity curves)

Peak velocities occur later than peak height velocity in boys and girls

Strength “Maturity” not reached until late twenties

Resistance Training

Children can weight train with individual monitoring

Effort/Benefit ratio is very high prior to puberty

Training prior to puberty has lasting effect

Fibre Typing

Type I– Red (Slow Twitch Fibres)

Type II (IIa & IIb)– White (Fast Twitch Fibres)

High proportion Type I and undifferentiated Type IIc fibres during early and mid-childhood in comparison to adulthood

Little known about sex associated differences in fibre type distribution

Muscle fibre type composition in young dancers, even at the very beginning of their professional dance training, differs from that in the average individual and is characterised by a high % of type I fibres, similar to that found in 20 years old dancers.

This, together with the fact that detraining did not change the muscle fibre type composition, supports the idea that the high percentage of type I fibres in dancers is due to a selection of individuals with suitable muscle fibre composition to the dance profession rather than being an effect of training.

Dahlström, M. (Karolinska Institute)The dancer: Physical effort, muscle fibre types, and energy intake and expenditure